When a Picker is used in a CellEditTemplate, and when in edit mode, when I click on the arrow of the Picker - the picker does not open. It only opens of I click the text in the Picker.

        <DataTemplate x:Key="template1">
            <Picker ItemsSource="{Binding Item.Categories}" />
        </DataTemplate>

        <telerik:RadDataGrid x:Name="dataGrid"
                             AutoGenerateColumns="False">
            <telerik:RadDataGrid.Columns>
                <telerik:DataGridTextColumn PropertyName="Name" />
                <telerik:DataGridTextColumn PropertyName="Category" CellEditTemplate="{StaticResource template1}" />
            </telerik:RadDataGrid.Columns>
        </telerik:RadDataGrid>

A work-around is to not use a Picker, but use a RadComboBox.